1. Field of the Invention
The present invention relates to a method of manufacturing a rigid internal gear of a wave gear device and to a rigid internal gear manufactured by this method of manufacturing. In more detail, the present invention pertains to a method of manufacturing a rigid internal gear that is made lighter with no loss in strength, abrasion resistance, etc., and to such a rigid internal gear.
2. Related Art
A typical wave gear device includes a circular rigid internal gear, a cup-shaped flexible external gear disposed inside the rigid internal gear, and a wave generator fitted into the flexible external gear. The wave generator normally has an elliptical outline, and the flexible external gear is flexed elliptically so that the flexible external teeth on its outer circumferential surface are partially meshed with the internal teeth of the rigid internal gear. When the wave generator is rotated by a motor or the like, the engaging portions of both gears move in a circumferential direction. Since there is a difference in the number of teeth between the external teeth and the internal teeth, relative rotation occurs between the gears in accordance with the difference in the number of teeth. Accordingly, if one of the gears is fixed, a reduced speed rotational output can be obtained from the other gear.
A wave gear device is characterized by having a low number of parts and by being lighter than other types of reduction gear. However, to achieve a further significant reduction in weight, it is necessary for the rigid internal gear of the wave gear device to make lighter. In more detail, a rigid internal gear needs to be strong and abrasion resistant, and so is manufactured using a heavy material such as a steel or copper material. As a result, the rigid internal gear accounts for a large proportion of the weight of the components of the wave gear device. In particular, in cases where an attachment flange for attaching a motor or an output-side member is integrally formed or a housing is integrally formed, the device becomes considerably heavy, so that the advantages of a wave gear device being lightweight are lost.
For the above reason, it is conceivable to use a ring made of a steel or copper material with high strength and abrasion resistance to form the part of the rigid internal gear in which the internal teeth are formed (i.e., the part for which strength and abrasion resistant are required), to use a ring made of a lightweight material such as an aluminum alloy to form the remaining main body of the gear, and to form the rigid internal gear by joining these rings to integrate them into a single component.
In the above case, it is necessary for the join between the main gear ring and the tooth-forming ring to transmit a large amount of torque, so that the join has to be dependably secured using fixings such as bolts. When the rings are joined in this way, it is necessary to manufacture the main gear ring and the tooth-forming ring as thick members, so that it is difficult to achieve weight reductions. Since the rings are attached using bolts or the like, there is also a corresponding increase in weight due to such fixings.